That every Ray consider'd apart, is constant to it self in some degree
of Refrangibility, is sufficiently manifest out of what has been said.
Those Rays, which in the first Refraction, are at equal Incidences most
refracted, are also in the following Refractions at equal Incidences
most refracted; and so of the least refrangible, and the rest which have
any mean Degree of Refrangibility, as is manifest by the fifth, sixth,
seventh, eighth, and ninth Experiments. And those which the first Time
at like Incidences are equally refracted, are again at like Incidences
equally and uniformly refracted, and that whether they be refracted
before they be separated from one another, as in the fifth Experiment,
or whether they be refracted apart, as in the twelfth, thirteenth and
fourteenth Experiments. The Refraction therefore of every Ray apart is
regular, and what Rule that Refraction observes we are now to shew.[E]
The late Writers in Opticks teach, that the Sines of Incidence are in a
given Proportion to the Sines of Refraction, as was explained in the
fifth Axiom, and some by Instruments fitted for measuring of
Refractions, or otherwise experimentally examining this Proportion, do
acquaint us that they have found it accurate. But whilst they, not
understanding the different Refrangibility of several Rays, conceived
them all to be refracted according to one and the same Proportion, 'tis
to be presumed that they adapted their Measures only to the middle of
the refracted Light; so that from their Measures we may conclude only
that the Rays which have a mean Degree of Refrangibility, that is, those
which when separated from the rest appear green, are refracted according
to a given Proportion of their Sines. And therefore we are now to shew,
that the like given Proportions obtain in all the rest. That it should
be so is very reasonable, Nature being ever conformable to her self; but
an experimental Proof is desired. And such a Proof will be had, if we
can shew that the Sines of Refraction of Rays differently refrangible
are one to another in a given Proportion when their Sines of Incidence
are equal. For, if the Sines of Refraction of all the Rays are in given
Proportions to the Sine of Refractions of a Ray which has a mean Degree
of Refrangibility, and this Sine is in a given Proportion to the equal
Sines of Incidence, those other Sines of Refraction will also be in
given Proportions to the equal Sines of Incidence. Now, when the Sines
of Incidence are equal, it will appear by the following Experiment, that
the Sines of Refraction are in a given Proportion to one another.
[Illustration: FIG. 26.]
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